Deficiency of fumarylacetoacetate hydrolase (FAH) causes tyrosinemia type I (hepatorenal tyrosinemia, HT 1) in humans and is characterized by severe liver dysfunction, renal tubular damage and liver cancer. In the murine model of HT 1 hepatocytes stably expressing Fah have a strong selective growth advantage. Fah mutant mice are therefore uniquely suited for the study of the biology of integrating gene therapy vectors in the liver. We have succeeded in the synthesis of 2 potent small inhibitors of Fah, which work in mice in vivo. Because of the powerful selection found in genetic Fah deficiency, pharmacological inhibition of Fah may be a useful strategy for the in vivo selection of genetically modified hepatocytes. FAH deficient humans and mice have a high propensity to the development of hepatocarcinoma. Our recent studies demonstrate that oxidative stress induced by fumarylacetoacetate (FAA) creates generalized apoptosis resistance in Fah mutant hepatocytes. Apoptosis resistance may therefore be an important early factor in the evolution of HCC in HT 1. The overall theme of this application is the utilization of the tyrosine catabolic pathway for in vivo metabolic selection of genetically modified epithelial cells in the liver. The feasibility and biology of various strategies for hepatic gene therapy will be explored. In aim 1, we will test in vivo gene targeting in the liver using AAV vectors. In aim 2, we will use small molecule inhibitors of Fah to select for genetically modified hepatocytes in vivo. In aim 3, the pathophysiology of hepatic cancer in HT 1 and strategies for its chemoprevention will be studied. [unreadable] [unreadable]